Imaging lens

ABSTRACT

An imaging lens includes a first lens group having positive refractive power and a second lens group having negative refractive power, arranged in this order from an object side to an image plane side. The first lens group includes a first lens, a second lens, and a third lens. The second lens group includes a fourth lens, a fifth lens having negative refractive power, and a sixth lens. The third lens has a concave surface facing the image plane side near an optical axis thereof. The sixth lens has a concave surface facing the object side near an optical axis thereof. The fifth lens has a specific Abbe&#39;s number.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of a prior application Ser. No.15/883,160, filed on Jan. 30, 2018, pending, which is a continuationapplication of a prior application Ser. No. 15/237,760, filed on Aug.16, 2016, issued on Jul. 3, 2018 as U.S. Pat. No. 10,012,820, which is acontinuation application of a prior application Ser. No. 14/609,469,issued on Sep. 20, 2016 as U.S. Pat. No. 9,448,387.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image ofan object on an imaging element such as a CCD sensor and a CMOS sensor.In particular, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a camera to be builtin a cellular phone, a portable information terminal, or the like, adigital still camera, a security camera, a vehicle onboard camera, and anetwork camera.

In these years, in place of cellular phones that are intended mainly formaking phone calls, so-called “smartphones”, i.e., multifunctionalcellular phones which can run various application software as well as avoice call function, have been more widely used. When applicationsoftware is run on smartphones, it is possible to achieve functions suchas those of digital still cameras and car navigation systems on thesmartphones. In order to achieve those various functions, most models ofsmartphones include cameras similar to cellular phones.

Generally speaking, product groups of such smartphones are oftencomposed according to specifications for beginners to advanced users.Among them, an imaging lens to be mounted in a product designed for theadvanced users is required to have a high-resolution lens configurationso as to be also applicable to a high pixel count imaging element ofthese years.

As a method of attaining the high-resolution imaging lens, there hasbeen a method of increasing the number of lenses that compose theimaging lens. However, the increase of the number of lenses easilycauses an increase in the size of the imaging lens. Therefore, the lensconfiguration having a large number of lenses has a disadvantage interms of mounting in a small-sized camera such as the above-describedsmartphones. For this reason, an imaging lens has been developed so asto restrain the number of lenses as small as possible. However, withrapid advancement in achieving the higher pixel count of an imagingelement in these days, an imaging lens has been developed so as toattain higher resolution rather than a shorter total track length of theimaging lens. For example, conventionally, it has been typical to mounta camera unit, which includes an imaging lens and an imaging element, inthe smartphone. There has also been an attempt to attach a separatecamera unit onto a smartphone, whereby it is possible to obtain imagesequivalent to those of digital still cameras.

In case of a lens configuration composed of six lenses, due to the largenumber of lenses of the imaging lens, it is somewhat difficult to reducethe size of the imaging lens. However, because of high flexibility indesign, it has potential to attain satisfactory correction ofaberrations and downsizing in a balanced manner. For example, as theimaging lens having the six-lens configuration as described above, animaging lens described in Patent Reference has been known.

Patent Reference: Japanese Patent Application Publication No.2013-195587

The imaging lens described in Patent Reference includes a first lensthat is positive and directs a convex surface thereof to an object side,a second lens that is negative and directs a concave surface thereof toan image plane side, a third lens that is negative and directs a concavesurface thereof to the object side, a fourth and fifth lenses that arepositive and direct convex surfaces thereof to the image plane side, anda sixth lens that is negative and directs a concave surface thereof tothe object side. According to the conventional imaging lens of PatentReference, by satisfying conditional expressions of a ratio between afocal length of the first lens and a focal length of the third lens anda ratio between a focal length of the second lens and a focal length ofthe whole lens system, it is achievable to satisfactorily correct adistortion and a chromatic aberration.

Each year, functions and sizes of cellular phones and smartphones aregetting higher and smaller, and the level of a small size required foran imaging lens is even higher than before. In case of the imaging lensof Patent Reference, since a distance from an object-side surface of afirst lens to an image plane of an imaging element is long, there is alimit by itself to achieve satisfactory correction of aberrations whiledownsizing the imaging lens to satisfy the above-described demands.

Here, such a problem is not specific to the imaging lens to be mountedin cellular phones and smartphones. Rather, it is a common problem evenfor an imaging lens to be mounted in a relatively small camera such asdigital still cameras, portable information terminals, security cameras,vehicle onboard cameras, and network cameras.

In view of the above-described problems in conventional techniques, anobject of the present invention is to provide an imaging lens that canattain both downsizing thereof and satisfactory aberration correction.

Further objects and advantages of the present invention will be apparentfrom the following description of the invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to a firstaspect of the present invention, an imaging lens includes a first lensgroup having positive refractive power;

and a second lens group having negative refractive power, arranged inthe order from an object side to an image plane side. The first lensgroup includes a first lens having positive refractive power, a secondlens having positive refractive power, and a third lens having negativerefractive power. The second lens group includes a fourth lens havingpositive refractive power, a fifth lens, and a sixth lens havingnegative refractive power.

According to the first aspect of the present invention, when the firstlens has a focal length f1, the second lens has a focal length f2, thefirst lens has an Abbe's number νd1, the second lens has an Abbe'snumber νd2, and the third lens has an Abbe's number νd3, the imaginglens of the present invention satisfies the following conditionalexpressions (1) to (4):

0.3<f1/f2<0.9  (1)

40<νd1<75  (2)

40<νd2<75  (3)

15<νd3<35  (4)

According to the first aspect of the present invention, the first lensgroup is composed of three lenses, whose refractive powers are arrangedin the order of positive-positive-negative. Those three lenses arerespectively made of lens materials that satisfy the conditionalexpressions (2) through (4), and the first and the second lenses and thethird lens are a combination of low dispersion materials and a highdispersion material. With such arrangement of refractive powers and theorder of Abbe's numbers of those lenses, in the first lens group, it isachievable to suitably restrain generation of chromatic aberration andsatisfactorily correct the chromatic aberration, if generated.

According to the first aspect of the present invention, positiverefractive power is shared between the two lenses, the first lens andthe second lens. Therefore, it is achievable to restrain the refractivepowers of the first lens and the second lens to be relatively weak, andit is achievable to suitably downsize the imaging lens whilesatisfactorily correcting aberrations.

As shown in the conditional expression (1), the first lens and thesecond lens having positive refractive powers in the first lens groupare formed such that the first lens has stronger refractive power thanthat of the second lens. Therefore, the first lens, which is the lensclosest to the object in the first lens group, has strong positiverefractive power, so that it is achievable to more suitably downsize theimaging lens.

When the imaging lens satisfies the conditional expression (1), it isalso achievable to restrain an astigmatism, a chromatic aberration, anda field curvature respectively within preferred ranges in a balancedmanner, while downsizing the imaging lens. When the value exceeds theupper limit of “0.9”, the first lens has weak refractive power relativeto that of the second lens. Therefore, although it is advantageous tosecure a back focal length, it is difficult to downsize the imaginglens.

In addition, since an astigmatic difference increases, it is difficultto correct the astigmatism, and it is difficult to obtain satisfactoryimage-forming performance. On the other hand, when the value is belowthe lower limit of “0.3”, the first lens has strong refractive powerrelative to that of the second lens, so that it is advantageous fordownsizing of the imaging lens. However, the back focal length is short,and it is difficult to secure space to dispose an insert such as aninfrared cutoff filter.

Moreover, in the astigmatism, a sagittal image surface curves to theobject side, and a chromatic aberration of magnification for an off-axislight flux is excessively corrected (an image-forming point at a shortwavelength moves in a direction to be away from an optical axis relativeto an image-forming point at a reference wavelength) at the periphery ofthe image. Therefore, it is difficult to obtain satisfactoryimage-forming performance.

According to a second aspect of the present invention, when the wholelens system has a focal length f, the first lens group has a focallength F1, the imaging lens having the above-described configurationpreferably satisfies the following conditional expression (5):

0.8<F1/f<1.2  (5)

When the imaging lens satisfies the conditional expression (5), it isachievable to restrain the chromatic aberration and the astigmatismwithin preferable ranges, while downsizing the imaging lens. Inaddition, when the imaging lens satisfies the conditional expression(5), it is also achievable to restrain an incident angle of a light beamemitted from the imaging lens to the image plane within the range ofchief ray angle (CRA). As is well known, a so-called chief ray angle(CRA) is set in advance for an imaging element such as a CCD sensor or aCMOS sensor, i.e., a range of an incident angle of a light beam that canbe taken in the sensor. By restraining the incident angle of a lightbeam emitted from the imaging lens to the image plane within the rangeof CRA, it is possible to suitably restrain generation of shading, whichis a phenomenon of becoming dark on the image periphery.

When the value exceeds the upper limit of “1.2” in the conditionalexpression (5), the first lens group has weak refractive power relativeto that of the whole lens system. Therefore, although it is advantageousfor correction of an axial chromatic aberration, it is difficult todownsize the imaging lens. Moreover, the astigmatic difference increasesat the periphery of the image, so that it is difficult to obtainsatisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “0.8”, thefirst lens group has strong refractive power relative to that of thewhole lens system, so that it is advantageous for downsizing of theimaging lens. However, the axial chromatic aberration is insufficientlycorrected (a focal position at a short wavelength moves to the objectside relative to a focal position at a reference wavelength), and achromatic aberration of magnification is excessively corrected.Moreover, an image-forming surface curves to the object side at imageperiphery, i.e., the field curvature is insufficiently corrected, sothat it is difficult to obtain satisfactory image-forming performance.Furthermore, it is also difficult to restrain the incident angle of alight beam emitted from the imaging lens to the image plane within therange of CRA.

According to a third aspect of the present invention, when the wholelens system has a focal length f, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (6):

1.0<f2/f<2.0  (6)

When the imaging lens satisfies the conditional expression (6), it ispossible to restrain the chromatic aberration and the astigmatism withinpreferred ranges, while downsizing the imaging lens. When the valueexceeds the upper limit of “2.0”, the second lens has weak refractivepower relative to that of the whole lens system. Therefore, the firstlens has relatively strong refractive power in the first lens group, andit is advantageous for downsizing of the imaging lens. However, theaxial chromatic aberration is insufficiently corrected, and thechromatic aberration of magnification for an off-axis light flux at theperiphery of the image is excessively corrected, so that it is difficultto obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “1.0”, thesecond lens has strong refractive power relative to that of the wholelens system. Therefore, although it is advantageous for securing theback focal length, the astigmatic difference increases at the imageperiphery, and it is difficult to obtain satisfactory image-formingperformance.

According to a fourth aspect of the present invention, when the thirdlens has a focal length f3, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(7):

−1.2<f1/f3<−0.4  (7)

When the imaging lens satisfies the conditional expression (7), it isachievable to satisfactorily correct the chromatic aberration, the fieldcurvature, and the astigmatism. When the value exceeds the upper limitof “−0.4”, the first lens has strong positive refractive power relativeto the negative refractive power of the third lens. As a result, theaxial chromatic aberration is insufficiently corrected and the chromaticaberration of magnification is excessively corrected. Moreover, thefield curvature is insufficiently corrected at the periphery of theimage, and it is difficult to obtain satisfactory image-formingperformance.

On the other hand, when the value is below the lower limit of “−1.2”,the first lens has weak positive refractive power relative to thenegative refractive power of the third lens. Therefore, in theastigmatism, a tangential image surface curves to the image plane sideand the astigmatic difference increases, so that also in this case, itis difficult to obtain satisfactory image-forming performance.

According to a fifth aspect of the present invention, when the firstlens group has a focal length F1, the second lens group has a focallength F2, the imaging lens having the above-described configurationpreferably satisfies the following conditional expression (8):

−12<F2/F1<−1.5  (8)

When the imaging lens satisfies the conditional expression (8), it isachievable to restrain the astigmatism, the field curvature, and thechromatic aberration within preferred ranges in a balanced manner, whiledownsizing the imaging lens. When the value exceeds the upper limit of“−1.5”, it is advantageous for downsizing of the imaging lens. However,the axial chromatic aberration is insufficiently corrected, and thechromatic aberration of magnification for an off-axis light flux at theperiphery of the image is excessively corrected. Moreover, theastigmatic difference increases, and the field curvature isinsufficiently corrected, so that it is difficult to obtain satisfactoryimage-forming performance.

On the other hand, when the value is below the lower limit of “−12”,although it is easy to secure the back focal length, it is difficult todownsize the imaging lens. In addition, the astigmatic differenceincreases and the image-forming surface curves to the image plane side,i.e., the field curvature is excessively corrected. As a result, it isdifficult to obtain satisfactory image-forming performance.

According to a sixth aspect of the present invention, when the wholelens system has a focal length f, a distance on an optical axis betweenthe third lens and the fourth lens is D34, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (9):

0.05<D34/f<0.35  (9)

When the imaging lens satisfies the conditional expression (9), it isachievable to restrain the distortion, the astigmatism, and the fieldcurvature within preferred ranges in a balanced manner, whilerestraining the incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. When the value exceedsthe upper limit of “0.35”, although it is easy to restrain the incidentangle within the range of CRA, it is difficult to secure the back focallength. Moreover, the plus distortion increases, and the field curvatureis excessively corrected, so that it is difficult to obtain satisfactoryimage-forming performance.

On the other hand, when the value is below the lower limit of “0.05”,the axial chromatic aberration is insufficiently corrected. In addition,the field curvature is insufficiently corrected and the astigmaticdifference increases, so that it is difficult to obtain satisfactoryimage-forming performance. Furthermore, it is difficult to restrain theincident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA.

According to a seventh aspect of the present invention, when the fourthlens has an Abbe's number νd4, the fifth lens has an Abbe's number νd5,and the sixth lens has an Abbe's number νd6, in order to moresatisfactorily correct the chromatic aberration, the imaging lens havingthe above-described configuration preferably satisfies the followingconditional expressions (10) through (12):

15<νd4<35  (10)

40<νd5<75  (11)

40<νd6<75  (12)

According to an eighth aspect of the present invention, when the fifthlens has negative refractive power, the fourth lens has a focal lengthf4, and the fifth lens has a focal length f5, the imaging lens havingthe above-described configuration preferably satisfies the followingconditional expression (13):

−15<f5/f4<−5  (13)

When the imaging lens satisfies the conditional expression (13), it isachievable to satisfactorily correct the chromatic aberration ofmagnification and the field curvature. When the value exceeds the upperlimit of “−5”, the fifth lens has strong negative refractive powerrelative to the positive refractive power of the fourth lens. As aresult, the chromatic aberration of magnification for an off-axis lightflux at the periphery of the image is excessively corrected and thefield curvature is insufficiently corrected. Therefore, it is difficultto obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−15”, thefifth lens has weak negative refractive power relative to the positiverefractive power of the fourth lens. In order to satisfactorily correctthe aberrations, it is necessary to increase the refractive power of thesixth lens, which has negative refractive power similar to the fifthlens in the second lens group. In this case, however, although it isadvantageous for correction of the field curvature, the chromaticaberration of magnification is excessively corrected at the periphery ofthe image, so that it is difficult to obtain satisfactory image-formingperformance.

According to a ninth aspect of the present invention, when the wholelens system has a focal length f, a composite focal length of the fifthlens and the sixth lens is f56, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (14):

−3<f56/f<−0.8  (14)

When the imaging lens satisfies the conditional expression (14), it isachievable to restrain the chromatic aberration, the distortion, and theastigmatism within preferred ranges in a balanced manner, whiledownsizing the imaging lens. When the value exceeds the upper limit of“−0.8”, the second lens group has relatively strong negative refractivepower, which is advantageous for downsizing of the imaging lens.However, the plus distortion increases and the chromatic aberration ofmagnification is excessively corrected at the periphery of the image, sothat it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−3”,although it is easy to secure the back focal length, it is difficult todownsize the imaging lens. Moreover, the minus distortion increases andthe astigmatic difference increases, so that it is difficult to obtainsatisfactory image-forming performance.

According to a tenth aspect of the present invention, when the sixthlens has a focal length f6 and a composite focal length of the fifthlens and the sixth lens is f56, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (15):

0.7<f6/f56<1.2  (15)

When the imaging lens satisfies the conditional expression (15), it isachievable to restrain the chromatic aberration, the field curvature,and the distortion within preferred ranges in a balanced manner. Asshown in the conditional expression (15), according to the imaging lensof the present invention, the negative refractive power of the secondlens group is mostly made up by the sixth lens. The fifth lens has veryweak refractive power relative to that of the sixth lens. With suchconfiguration, the fifth lens can contribute to fine correction of theaberrations, and the sixth lens can contribute to suitably restrainingof the incident angle to the image plane within the range of CRA as wellas correction of the aberrations.

When the value exceeds the upper limit of “1.2”, although it isadvantageous for correction of the axial chromatic aberration, the minusdistortion increases. Moreover, the field curvature is insufficientlycorrected and the chromatic aberration of magnification is excessivelycorrected, so that it is difficult to obtain satisfactory image-formingperformance. On the other hand, when the value is below the lower limitof “0.7”, although it is easy to correct the distortion, the axialchromatic aberration is insufficiently corrected, and it is difficult toobtain satisfactory image-forming performance.

According to an eleventh aspect of the present invention, when the wholelens system has a focal length f, the sixth lens has a focal length f6,the imaging lens having the above-described configuration preferablysatisfies the following conditional expression (16):

−3.5<f6/f<−0.5  (16)

When the imaging lens satisfies the conditional expression (16), it isachievable to restrain the chromatic aberration, the distortion, and theastigmatism within preferred ranges in a balanced manner, whiledownsizing the imaging lens. In addition, when the imaging lenssatisfies the conditional expression (16), it is also achievable torestrain the incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. When the value exceedsthe upper limit of “−0.5”, it is advantageous for correction of theaxial chromatic aberration. However, the plus distortion increases andthe chromatic aberration of magnification for an off-axis light flux atthe periphery of the image is excessively corrected, so that it isdifficult to obtain satisfactory image-forming performance. In addition,it is difficult to restrain the incident angle of a light beam emittedfrom the imaging lens to the image plane within the range of CRA.

On the other hand, when the value is below the lower limit of “−3.5”,although it is easy to restrain the incident angle to the image planewithin the range of CRA, the minus distortion increases and thechromatic aberration of magnification for an off-axis light flux at theperiphery of the image is insufficiently corrected, so that it isdifficult to obtain satisfactory image-forming performance.

According to the imaging lens of the present invention, it is possibleto provide a small-sized imaging lens that is especially suitable formounting in a small-sized camera, while having high resolution withsatisfactory correction of aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 1 according to an embodiment of thepresent invention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 2 according to the embodiment of thepresent invention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 3 according to the embodiment of thepresent invention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 4 according to the embodiment ofthe present invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 5 according to the embodiment ofthe present invention;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 13;

FIG. 15 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 13;

FIG. 16 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 6 according to the embodiment ofthe present invention;

FIG. 17 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 16;

FIG. 18 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 16;

FIG. 19 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 7 according to the embodiment ofthe present invention;

FIG. 20 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 19; and

FIG. 21 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 19.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, an embodiment of thepresent invention will be fully described.

FIGS. 1, 4, 7, 10, 13, 16, and 19 are schematic sectional views of theimaging lenses in Numerical Data Examples 1 to 7 according to theembodiment, respectively. Since the imaging lenses in those NumericalData Examples have the same basic configuration, the lens configurationof the embodiment will be described with reference to the illustrativesectional view of Numerical Data Example 1.

As shown in FIG. 1, according to the embodiment, the imaging lensincludes a first lens group G1 having positive refractive power, and asecond lens group G2 having negative refractive power, arranged in theorder from an object side to an image plane side. Between the secondlens group G2 and an image plane IM of an imaging element, there isprovided a filter 10. The filter 10 is omissible.

The first lens group G1 includes a first lens L1 having positiverefractive power, an aperture stop ST, a second lens L2 having positiverefractive power, and a third lens L3 having negative refractive power,arranged in the order from the object side. According to the imaginglens of the embodiment, the aperture stop ST is provided on an imageplane-side surface of the first lens L1. Here, the position of theaperture stop ST is not limited to be between the first lens L1 and thesecond lens L2 as in the imaging lens of Numerical Data Example 1.

For example, it is also possible to dispose the aperture stop ST on theobject side of the first lens L1. In case of a so-called “frontaperture”-type lens configuration, in which the aperture stop ST isdisposed on the object side of the imaging lens, it is achievable toimprove the ease of lens assembly and reduce the manufacturing cost. Incase of the front aperture-type lens configuration, since it is alsorelatively easy to shorten a total track length of the imaging lens, thelens configuration is also effective for mounting in a portable devicesuch as cellular phones and smartphones that are popular in these years.

On the other hand, in case of a so-called “mid aperture”-type lensconfiguration, in which the aperture stop ST is disposed between thefirst lens L1 and the second lens L2 as in Numerical Data Example 1, aneffective diameter of the first lens L1 is large relative to the totaltrack length of the imaging lens. Therefore, the presence of the imaginglens in a camera is emphasized, and it is possible to appeal to users bythe luxurious impression, high lens performance, etc. as a part ofdesign of the camera.

In the first lens group G1, the first lens L1 is formed in a shape suchthat a curvature radius r1 of an object-side surface thereof is positiveand a curvature radius r2 of an image plane-side surface thereof isnegative, so as to have a shape of a biconvex lens near an optical axisX. The shape of the first lens L1 is not limited to the one in NumericalData Example 1. The first lens L1 can be formed in any shape, as long asthe curvature radius r1 of the object-side surface thereof is positive.More specifically, the first lens L1 can also be formed in a shape suchthat the curvature radius r2 is positive so as to have a shape of ameniscus lens directing a convex surface thereof to the object side nearthe optical axis X.

The second lens L2 is formed in a shape such that a curvature radius r3of an object-side surface and a curvature radius r4 of an imageplane-side surface thereof are both negative, so as to have a shape of ameniscus lens directing a concave surface thereof to the object sidenear the optical axis X. The shape of the second lens L2 is not limitedto the one in Numerical Data Example 1. The second lens L2 can be formedin any shape, as long as the curvature radius r4 of the image plane-sidesurface thereof is negative. More specifically, the second lens L2 canbe formed in a shape such that the curvature radius r3 of theobject-side surface thereof is positive so as to have a shape of abiconvex lens near the optical axis X. Here, generally speaking, whenthe first lens L1 has a shape of a meniscus lens directing a convexsurface thereof to the object side near the optical axis X, the shape ofthe second lens L2 is preferably biconvex near the optical axis X asdescribed above.

The third lens L3 is formed in a shape such that a curvature radius r5of an object-side surface thereof is negative and a curvature radius r6of an image plane-side surface thereof is positive, so as to have ashape of a biconcave lens near the optical axis X. The shape of thethird lens L3 is not limited to the one in Numerical Data Example 1, andcan be any as long as the curvature radius r6 of the image plane-sidesurface thereof is positive. More specifically, the third lens L3 canalso be formed in a shape such that the curvature radius r5 of theobject-side surface thereof is positive, i.e. a shape of a meniscus lensdirecting a convex surface thereof to the object side near the opticalaxis X. The second lens group G2 includes the fourth lens L4 havingpositive refractive power, the fifth lens L5 having negative or positiverefractive power, and the sixth lens L6 having negative refractivepower, arranged in the order from the object side.

In the second lens group G2, the fourth lens L4 is formed in a shapesuch that a curvature radius r7 of an object-side surface thereof and acurvature radius r8 of an image plane-side surface thereof are bothnegative, so as to have a shape of a meniscus lens directing a concavesurface thereof to the object side near the optical axis X. The fifthlens L5 is formed in a shape such that a curvature radius r9 of anobject-side surface thereof and a curvature radius r10 of an imageplane-side surface thereof are both positive, so as to have a shape of ameniscus lens directing a convex surface thereof to the object side nearthe optical axis X. The fifth lens L5 has the weakest refractive powerin the second lens group G2. The imaging lenses of Numerical DataExamples 1 to 6 are examples of a lens configuration, in which the fifthlens L5 has negative refractive power. The imaging lens of NumericalData Example 7 is an example, in which the fifth lens L5 has positiverefractive power.

The sixth lens L6 is formed in a shape such that a curvature radius r11of an object-side surface thereof is negative and a curvature radius r12of an image plane-side surface thereof is positive, so as to have ashape of a biconcave lens near the optical axis X. The shape of thesixth lens L6 is not limited to the one in Numerical Data Example 1, andcan be any as long as the curvature radius r12 of the image plane-sidesurface thereof is positive. More specifically, the sixth lens L6 canalso be formed in a shape such that the curvature radius r11 ispositive, so as to have a shape of a meniscus lens directing a convexsurface thereof to the object side near the optical axis X. NumericalData Examples 5 and 6 are examples, in which the shape of the sixth lensL6 is a meniscus lens directing a convex surface thereof to the objectside near the optical axis X.

The fifth lens L5 and the sixth lens L6 are formed such that theobject-side surfaces thereof and the image plane-side surfaces thereofare formed as aspheric surfaces and formed in shapes such that positiverefractive power becomes stronger toward the lens peripheries. Withthose shapes of the fifth lens L5 and the sixth lens L6, it isachievable to satisfactorily correct not only the axial chromaticaberration but also the off-axis chromatic aberration of magnification,and to suitably restrain the incident angle of a light beam emitted fromthe imaging lens to the image plane IM within the range of a chief rayangle (CRA).

According to the embodiment, the imaging lens satisfies the followingconditional expressions (1) to (16):

0.3<f1/f2<0.9  (1)

40<νd1<75  (2)

40<νd2<75  (3)

15<νd3<35  (4)

0.8<F1/f<1.2  (5)

1.0<f2/f<2.0  (6)

−1.2<f1/f3<−0.4  (7)

−12<F2/F1<−1.5  (8)

0.05<D34/f<0.35  (9)

15<νd4<35  (10)

40<νd5<75  (11)

40<νd6<75  (12)

−15<f5/f4<−5  (13)

−3<f56/f<−0.8  (14)

0.7<f6/f56<1.2  (15)

−3.5<f6/f<−0.5  (16)

In the above conditional expressions:

f: Focal length of a whole lens systemF1: Focal length of the first lens group G1F2: Focal length of the second lens group G2f1: Focal length of the first lens L1f2: Focal length of the second lens L2f3: Focal length of the third lens L3f4: Focal length of the fourth lens L4f5: Focal length of the fifth lens L5f6: Focal length of the sixth lens L6f56: Composite focal length of the fifth lens L5 and the sixthlens L6D34: Distance on the optical axis X between the third lens L3and the fourth lens L4νd1: Abbe's number of the first lens L1νd2: Abbe's number of the second lens L2νd3: Abbe's number of the third lens L3νd4: Abbe's number of the fourth lens L4νd5: Abbe's number of the fifth lens L5νd6: Abbe's number of the sixth lens L6

Here, it is not necessary to satisfy all of the conditional expressions,and it is achievable to obtain an effect corresponding to the respectiveconditional expression when any single one of the conditionalexpressions is individually satisfied.

In the embodiment, all lens surfaces are formed as an aspheric surface.When the aspheric shapes applied to the lens surfaces have an axis Z ina direction of the optical axis X, a height H in a directionperpendicular to the optical axis X, a conic constant k, and asphericcoefficients A₄, A₆, A₈, A₁₀, A₁₂, A₁₄, and A₁₆, the aspheric shapes ofthe lens surfaces are expressed as follows:

$\begin{matrix}{Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {( {k + 1} )\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}} + {A_{14}H^{14}} + {A_{16}H^{16}}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each Numerical Data Example, f represents a focallength of the whole lens system, Fno represents an F-number, and ωrepresents a half angle of view, respectively. In addition, i representsa surface number counted from the object side, r represents a curvatureradius, d represents a distance on the optical axis between lenssurfaces (surface spacing), nd represents a refractive index, and νdrepresents an Abbe's number, respectively. Here, aspheric surfaces areindicated with surface numbers i affixed with * (asterisk).

Numerical Data Example 1

Basic data are shown below.

f = 4.33 mm, Fno = 2.2, ω = 38.2° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* 2.952 0.545 1.5346 56.1 (=νd1) 2* (Stop)−5.004 0.135  3* −6.123 0.687 1.5346 56.1 (=νd2)  4* −2.024 0.039  5*−6.319 0.276 1.6355 24.0 (=νd3)  6* 4.221 0.574 (=D34)  7* −2.502 0.6001.6142 26.0 (=νd4)  8* −1.965 0.089  9* 2.907 0.651 1.5346 56.1 (=νd5)10* 2.542 0.468 11* −14.797 0.385 1.5346 56.1 (=νd6) 12* 3.222 0.100 13∞ 0.300 1.5168 64.2 14 ∞ 0.594 (Image ∞ plane) Aspheric Surface DataFirst Surface k = 0.000, A₄ = −5.137E−02, A₆ = −2.984E−03, A₈ =−4.195E−02, A₁₀ = 2.210E−02, A₁₂ = 1.049E−02, A₁₄ = −2.184E−02, A₁₆ =9.763E−03 Second Surface k = 0.000, A₄ = −2.310E−02, A₆ = 2.771E−03, A₈= −2.867E−02, A₁₀ = 1.898E−02, A₁₂ = 2.730E−02, A₁₄ = −5.132E−02, A₁₆ =2.649E−02 Third Surface k = 0.000, A₄ = 4.012E−02, A₆ = −2.211E−02, A₈ =2.330E−02, A₁₀ = −5.307E−02, A₁₂ = 2.119E−02, A₁₄ = −5.489E−03, A₁₆ =3.489E−03 Fourth Surface k = 0.000, A₄ = −1.972E−02, A₆ = −2.138E−02, A₈= 2.704E−03, A₁₀ = 4.225E−04, A₁₂ = −1.088E−03, A₁₄ = 7.878E−04, A₁₆ =−8.316E−04 Fifth Surface k = 0.000, A₄ = −1.503E−01, A₆ = 4.707E−02, A₈= 2.472E−02, A₁₀ = −1.528E−04, A₁₂ = 1.313E−02, A₁₄ = −1.462E−02, A₁₆ =3.514E−03 Sixth Surface k = 0.000, A₄ = −8.746E−02, A₆ = 4.078E−02, A₈ =9.095E−04, A₁₀ = 2.146E−03, A₁₂ = −6.727E−03, A₁₄ = 6.470E−03, A₁₆ =−1.933E−03 Seventh Surface k = 0.000, A₄ = 1.303E−01, A₆ = −1.218E−01,A₈ = 8.443E−02, A₁₀ = −5.973E−02, A₁₂ = 9.218E−03, A₁₄ = 5.782E−03, A₁₆= −1.730E−03 Eighth Surface k = 0.000, A₄ = 3.252E−02, A₆ = 1.663E−02,A₈ = −1.486E−02, A₁₀ = 1.026E−03, A₁₂ = −1.109E−03, A₁₄ = 2.840E−04, A₁₆= 2.827E−04 Ninth Surface k = 0.000, A₄ = −1.577E−01, A₆ = 5.096E−02, A₈= −1.989E−02, A₁₀ = 2.009E−03, A₁₂ = −4.004E−04, A₁₄ = 1.630E−04, A₁₆ =−4.771E−06 Tenth Surface k = 0.000, A₄ = −1.226E−01, A₆ = 2.829E−02, A₈= −8.356E−03, A₁₀ = 9.028E−04, A₁₂ = 2.097E−04, A₁₄ = −5.171E−05, A₁₆ =2.624E−06 Eleventh Surface k = 0.000, A₄ = −8.750E−02, A₆ = 1.549E−02,A₈ = 2.275E−04, A₁₀ = 2.362E−05, A₁₂ = −5.834E−05, A₁₄ = 6.461E−06, A₁₆= −1.641E−07 Twelfth Surface k = 0.000, A₄ = −9.992E−02, A₆ = 2.511E−02,A₈ = −3.639E−03, A₁₀ = 1.960E−04, A₁₂ = 8.694E−06, A₁₄ = −1.728E−06, A₁₆= 6.770E−08 f1 = 3.56 mm f2 = 5.34 mm f3 = −3.94 mm f4 = 10.46 mm f5 =−100.09 mm f6 = −4.91 mm f56 = −5.02 mm F1 = 4.43 mm F2 = −11.25 mm Thevalues of the respective conditional expressions are as follows: f1/f2 =0.67 F1/f = 1.02 f2/f = 1.23 f1/f3 = −0.90 F2/F1 = −2.54 D34/f = 0.13f5/f4 = −9.57 f56/f = −1.16 f6/f56 = 0.98 f6/f = −1.13

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.34 mm, and downsizing ofthe imaging lens is attained.

FIG. 2 shows a lateral aberration that corresponds to a ratio H of eachimage height to the maximum image height (hereinafter referred to as“image height ratio H”), which is divided into a tangential directionand a sagittal direction (The same is true for FIGS. 5, 8, 11, 14, 17,and 20). Furthermore, FIG. 3 shows a spherical aberration (mm),astigmatism (mm), and a distortion (%), respectively. In the astigmatismdiagram, an aberration on a sagittal image surface S and an aberrationon a tangential image surface T are respectively indicated (The same istrue for FIGS. 6, 9, 12, 15, 18, and 21). As shown in FIGS. 2 and 3,according to the imaging lens of Numerical Data Example 1, theaberrations are satisfactorily corrected.

Numerical Data Example 2

Basic data are shown below.

f = 4.41 mm, Fno = 2.2, ω = 37.7° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* 2.777 0.614 1.5346 56.1 (=νd1) 2* (Stop)−4.101 0.109  3* −4.097 0.608 1.5346 56.1 (=νd2)  4* −2.280 0.059  5*−10.179 0.269 1.6355 24.0 (=νd3)  6* 3.839 0.605 (=D34)  7* −2.637 0.6001.6142 26.0 (=νd4)  8* −2.032 0.068  9* 2.867 0.683 1.5346 56.1 (=νd5)10* 2.495 0.447 11* −20.730 0.404 1.5346 56.1 (=νd6) 12* 3.320 0.100 13∞ 0.300 1.5168 64.2 14 ∞ 0.631 (Image ∞ plane) Aspheric Surface DataFirst Surface k = 0.000, A₄ = −4.774E−02, A₆ = −5.967E−03, A₈ =−4.342E−02, A₁₀ = 2.143E−02, A₁₂ = 1.020E−02, A₁₄ = −2.197E−02, A₁₆ =9.790E−03 Second Surface k = 0.000, A₄ = −2.032E−02, A₆ = 2.834E−03, A₈= −2.865E−02, A₁₀ = 1.925E−02, A₁₂ = 2.745E−02, A₁₄ = −5.163E−02, A₁₆ =2.546E−02 Third Surface k = 0.000, A₄ = 4.925E−02, A₆ = −1.824E−02, A₈ =2.665E−02, A₁₀ = −5.057E−02, A₁₂ = 2.260E−02, A₁₄ = −5.001E−03, A₁₆ =3.403E−03 Fourth Surface k = 0.000, A₄ = −2.168E−02, A₆ = −2.133E−02, A₈= 2.034E−03, A₁₀ = 3.010E−04, A₁₂ = −6.787E−04, A₁₄ = 1.226E−03, A₁₆ =−6.058E−04 Fifth Surface k = 0.000, A₄ = −1.521E−01, A₆ = 4.348E−02, A₈= 2.402E−02, A₁₀ = −4.407E−06, A₁₂ = 1.329E−02, A₁₄ = −1.460E−02, A₁₆ =3.374E−03 Sixth Surface k = 0.000, A₄ = −8.509E−02, A₆ = 4.221E−02, A₈ =2.434E−03, A₁₀ = 3.010E−03, A₁₂ = −6.542E−03, A₁₄ = 6.412E−03, A₁₆ =−1.976E−03 Seventh Surface k = 0.000, A₄ = 1.270E−01, A₆ = −1.213E−01,A₈ = 8.665E−02, A₁₀ = −5.967E−02, A₁₂ = 8.844E−03, A₁₄ = 5.705E−03, A₁₆= −1.522E−03 Eighth Surface k = 0.000, A₄ = 2.921E−02, A₆ = 1.518E−02,A₈ = −1.487E−02, A₁₀ = 1.128E−03, A₁₂ = −1.116E−03, A₁₄ = 2.385E−04, A₁₆= 2.474E−04 Ninth Surface k = 0.000, A₄ = −1.551E−01, A₆ = 5.179E−02, A₈= −1.993E−02, A₁₀ = 2.061E−03, A₁₂ = −3.480E−04, A₁₄ = 1.802E−04, A₁₆ =−2.742E−06 Tenth Surface k = 0.000, A₄ = −1.238E−01, A₆ = 2.829E−02, A₈= −8.372E−03, A₁₀ = 8.973E−04, A₁₂ = 2.093E−04, A₁₄ = −5.173E−05, A₁₆ =2.629E−06 Eleventh Surface k = 0.000, A₄ = −8.760E−02, A₆ = 1.537E−02,A₈ = 2.183E−04, A₁₀ = 2.367E−05, A₁₂ = −5.823E−05, A₁₄ = 6.483E−06, A₁₆= −1.607E−07 Twelfth Surface k = 0.000, A₄ = −9.780E−02, A₆ = 2.510E−02,A₈ = −3.643E−03, A₁₀ = 1.965E−04, A₁₂ = 8.839E−06, A₁₄ = −1.712E−06, A₁₆= 6.852E−08 f1 = 3.20 mm f2 = 8.61 mm f3 = −4.35 mm f4 = 10.46 mm f5 =−99.88 mm f6 = −5.32 mm f56 = −5.45 mm F1 = 4.62 mm F2 = −13.81 mm Thevalues of the respective conditional expressions are as follows: f1/f2 =0.37 F1/f = 1.05 f2/f = 1.95 f1/f3 = −0.73 F2/F1 = −2.99 D34/f = 0.14f5/f4 = −9.55 f56/f = −1.24 f6/f56 = 0.98 f6/f = −1.21

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.39 mm, and downsizing ofthe imaging lens is attained.

FIG. 5 shows a lateral aberration that corresponds to the image heightratio H, and FIG. 6 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively, of the imaging lens of NumericalData Example 2. As shown in FIGS. 5 and 6, according to the imaging lensof Numerical Data Example 2, the aberrations are also satisfactorilycorrected.

Numerical Data Example 3

Basic data are shown below.

f = 4.29 mm, Fno = 2.2, ω = 38.5° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* 3.463 0.516 1.5346 56.1 (=νd1) 2* (Stop)−5.004 0.134  3* −9.355 0.720 1.5346 56.1 (=νd2)  4* −1.972 0.039  5*−5.603 0.279 1.6355 24.0 (=νd3)  6* 4.021 0.561 (=D34)  7* −2.442 0.6001.6142 26.0 (=νd4)  8* −1.891 0.091  9* 2.907 0.734 1.5346 56.1 (=νd5)10* 2.515 0.457 11* −16.090 0.427 1.5346 56.1 (=νd6) 12* 3.316 0.100 13∞ 0.300 1.5168 64.2 14 ∞ 0.584 (Image ∞ plane) Aspheric Surface DataFirst Surface k = 0.000, A₄ = −5.535E−02, A₆ = −1.917E−03, A₈ =−3.996E−02, A₁₀ = 2.333E−02, A₁₂ = 1.081E−02, A₁₄ = −2.193E−02, A₁₆ =9.435E−03 Second Surface k = 0.000, A₄ = −2.325E−02, A₆ = 2.571E−03, A₈= −2.870E−02, A₁₀ = 1.924E−02, A₁₂ = 2.771E−02, A₁₄ = −5.110E−02, A₁₆ =2.623E−02 Third Surface k = 0.000, A₄ = 3.729E−02, A₆ = −2.472E−02, A₈ =2.208E−02, A₁₀ = −5.372E−02, A₁₂ = 2.066E−02, A₁₄ = −5.735E−03, A₁₆ =3.828E−03 Fourth Surface k = 0.000, A₄ = −1.774E−02, A₆ = −2.120E−02, A₈= 2.435E−03, A₁₀ = 2.765E−04, A₁₂ = −1.088E−03, A₁₄ = 8.235E−04, A₁₆ =−8.619E−04 Fifth Surface k = 0.000, A₄ = −1.499E−01, A₆ = 4.691E−02, A₈= 2.405E−02, A₁₀ = −6.608E−04, A₁₂ = 1.293E−02, A₁₄ = −1.463E−02, A₁₆ =3.594E−03 Sixth Surface k = 0.000, A₄ = −9.285E−02, A₆ = 3.888E−02, A₈ =−1.381E−04, A₁₀ = 1.635E−03, A₁₂ = −6.993E−03, A₁₄ = 6.317E−03, A₁₆ =−2.025E−03 Seventh Surface k = 0.000, A₄ = 1.345E−01, A₆ = −1.210E−01,A₈ = 8.433E−02, A₁₀ = −5.928E−02, A₁₂ = 9.524E−03, A₁₄ = 5.904E−03, A₁₆= −1.696E−03 Eighth Surface k = 0.000, A₄ = 3.338E−02, A₆ = 1.778E−02,A₈ = −1.402E−02, A₁₀ = 1.194E−03, A₁₂ = −1.082E−03, A₁₄ = 2.965E−04, A₁₆= 2.943E−04 Ninth Surface k = 0.000, A₄ = −1.536E−01, A₆ = 5.270E−02, A₈= −2.085E−02, A₁₀ = 2.263E−03, A₁₂ = −2.503E−04, A₁₄ = 1.772E−04, A₁₆ =−2.603E−05 Tenth Surface k = 0.000, A₄ = −1.236E−01, A₆ = 2.837E−02, A₈= −8.354E−03, A₁₀ = 8.999E−04, A₁₂ = 2.096E−04, A₁₄ = −5.171E−05, A₁₆ =2.609E−06 Eleventh Surface k = 0.000, A₄ = −8.659E−02, A₆ = 1.540E−02,A₈ = 2.172E−04, A₁₀ = 2.345E−05, A₁₂ = −5.824E−05, A₁₄ = 6.479E−06, A₁₆= −1.620E−07 Twelfth Surface k = 0.000, A₄ = −9.707E−02, A₆ = 2.519E−02,A₈ = −3.641E−03, A₁₀ = 1.958E−04, A₁₂ = 8.746E−06, A₁₄ = −1.719E−06, A₁₆= 6.825E−08 f1 = 3.91 mm f2 = 4.52 mm f3 = −3.64 mm f4 = 9.64 mm f5 =−100.02 mm f6 = −5.10 mm f56 = −5.26 mm F1 = 4.62 mm F2 = −15.02 mm Thevalues of the respective conditional expressions are as follows: f1/f2 =0.87 F1/f = 1.08 f2/f = 1.05 f1/f3 = −1.07 F2/F1 = −3.25 D34/f = 0.13f5/f4 = −10.38 f56/f = −1.23 f6/f56 = 0.97 f6/f = −1.19

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.44 mm, and downsizing ofthe imaging lens is attained.

FIG. 8 shows a lateral aberration that corresponds to the image heightratio H, and FIG. 9 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively, of the imaging lens of NumericalData Example 3. As shown in FIGS. 8 and 9, according to the imaging lensof Numerical Data Example 3, the aberrations are also satisfactorilycorrected.

Numerical Data Example 4

Basic data are shown below.

f = 4.81 mm, Fno = 2.4, ω = 35.4° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* 5.003 0.467 1.5346 56.1 (=νd1) 2* (Stop)−3.110 0.054  3* −3.109 0.536 1.5346 56.1 (=νd2)  4* −2.013 0.037  5*−5.259 0.263 1.6355 24.0 (=νd3)  6* 11.775 1.520 (=D34)  7* −4.054 0.6001.6142 26.0 (=νd4)  8* −2.156 0.050  9* 2.732 0.403 1.5346 56.1 (=νd5)10* 2.446 0.561 11* −4.624 0.299 1.5346 56.1 (=νd6) 12* 3.817 0.100 13 ∞0.300 1.5168 64.2 14 ∞ 0.749 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = −6.785E−02, A₆ = −2.546E−02, A₈ = −5.158E−02,A₁₀= 2.913E−02, A₁₂ = 1.851E−02, A₁₄ = −3.233E−03, A₁₆ = −6.237E−03Second Surface k = 0.000, A₄ = −4.128E−02, A₆ = 2.090E−02, A₈ =−2.933E−02, A₁₀ = 1.718E−02, A₁₂ = 3.764E−02, A₁₄ = −4.577E−02, A₁₆ =1.311E−02 Third Surface k = 0.000, A₄ = 4.816E−02, A₆ = 1.829E−03, A₈ =4.290E−02, A₁₀ = −5.132E−02, A₁₂ = 2.010E−02, A₁₄ = −7.058E−03, A₁₆ =−9.970E−05 Fourth Surface k = 0.000, A₄ = 4.722E−02, A₆ = −5.688E−02, A₈= 1.907E−02, A₁₀ = 4.455E−03, A₁₂ = −1.312E−03, A₁₄ = −4.132E−04, A₁₆ =−1.302E−03 Fifth Surface k = 0.000, A₄ = −7.379E−02, A₆ = 2.892E−02, A₈= 1.504E−02, A₁₀ = −1.198E−03, A₁₂ = 1.372E−02, A₁₄ = −1.442E−02, A₁₆ =3.305E−03 Sixth Surface k = 0.000, A₄ = −8.338E−02, A₆ = 4.790E−02, A₈ =1.388E−03, A₁₀ = 4.645E−04, A₁₂ = −8.286E−03, A₁₄ = 6.082E−03, A₁₆ =−1.545E−03 Seventh Surface k = 0.000, A₄ = 9.206E−03, A₆ = −9.210E−02,A₈ = 1.030E−01, A₁₀ = −5.866E−02, A₁₂ = 6.350E−03, A₁₄ = 4.505E−03, A₁₆= −9.761E−04 Eighth Surface k = 0.000, A₄ = −2.031E−02, A₆ = 2.042E−02,A₈ = −1.129E−02, A₁₀ = 2.128E−03, A₁₂ = −8.758E−04, A₁₄ = 1.371E−04, A₁₆= 5.360E−05 Ninth Surface k = 0.000, A₄ = −1.440E−01, A₆ = 6.840E−02, A₈= −3.271E−02, A₁₀ = 7.023E−03, A₁₂ = −5.822E−04, A₁₄ = 2.376E−05, A₁₆ =−8.656E−06 Tenth Surface k = 0.000, A₄ = −1.131E−01, A₆ = 2.256E−02, A₈= −8.356E−03, A₁₀ = 1.036E−03, A₁₂ = 2.169E−04, A₁₄ = −5.279E−05, A₁₆ =2.337E−06 Eleventh Surface k = 0.000, A₄ = −6.799E−02, A₆ = 1.453E−02,A₈ = 1.346E−04, A₁₀ = 2.693E−05, A₁₂ = −5.759E−05, A₁₄ = 6.511E−06, A₁₆= −1.691E−07 Twelfth Surface k = 0.000, A₄ = −8.500E−02, A₆ = 2.551E−02,A₈ = −3.947E−03, A₁₀ = 2.234E−04, A₁₂ = 9.589E−06, A₁₄ = −1.807E−06, A₁₆= 6.410E−08 f1 = 3.66 mm f2 = 9.13 mm f3 = −5.69 mm f4 = 6.69 mm f5 =−85.84 mm f6 = −3.86 mm f56 = −3.86 mm F1 = 5.08 mm F2 = −13.07 mm Thevalues of the respective conditional expressions are as follows: f1/f2 =0.40 F1/f = 1.06 f2/f = 1.90 f1/f3 = −0.64 F2/F1 = −2.57 D34/f = 0.32f5/f4 = −12.82 f56/f = −0.80 f6/f56 = 1.00 f6/f = −0.80

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.84 mm, and downsizing ofthe imaging lens is attained.

FIG. 11 shows a lateral aberration that corresponds to the image heightratio H, and FIG. 12 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively, of the imaging lens ofNumerical Data Example 4. As shown in FIGS. 11 and 12, according to theimaging lens of Numerical Data Example 4, the aberrations are alsosatisfactorily corrected.

Numerical Data Example 5

Basic data are shown below.

f = 4.37 mm, Fno = 2.2, ω = 37.9 ° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞  1* 3.241 0.516 1.5346 56.1 (=νd1) 2* (Stop)−4.985 0.126  3* −6.668 0.661 1.5346 56.1 (=νd2)  4* −2.148 0.039  5*−8.367 0.266 1.6355 24.0 (=νd3)  6* 3.795 0.537 (=D34)  7* −2.277 0.6001.6142 26.0 (=νd4)  8* −2.121 0.079  9* 2.976 1.046 1.5346 56.1 (=νd5)10* 2.478 0.323 11* 8.693 0.333 1.5346 56.1 (=νd6) 12* 3.718 0.100 13 ∞0.300 1.5168 64.2 14 ∞ 0.804 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = −5.240E−02, A₆ = −3.561E−03, A₈ = −4.105E−02,A₁₀ = 2.229E−02, A₁₂ = 1.015E−02, A₁₄ = −2.219E−02, A₁₆ = 9.826E−03Second Surface k = 0.000, A₄ = −2.277E−02, A₆ = 1.217E−03, A₈ =−2.958E−02, A₁₀ = 1.920E−02, A₁₂ = 2.777E−02, A₁₄ = −5.131E−02, A₁₆ =2.531E−02 Third Surface k = 0.000, A₄ = 4.057E−02, A₆ = −2.331E−02, A₈ =2.368E−02, A₁₀ = −5.191E−02, A₁₂ = 2.219E−02, A₁₄ = −5.065E−03, A₁₆ =3.361E−03 Fourth Surface k = 0.000, A₄ = −1.999E−02, A₆ = −2.216E−02, A₈= 2.362E−03, A₁₀ = 6.018E−04, A₁₂ = −7.646E−04, A₁₄ = 9.933E−04, A₁₆ =−7.473E−04 Fifth Surface k = 0.000, A₄ = −1.534E−01, A₆ = 4.334E−02, A₈= 2.356E−02, A₁₀ = −2.008E−04, A₁₂ = 1.323E−02, A₁₄ = −1.464E−02, A₁₆ =3.390E−03 Sixth Surface k = 0.000, A₄ = −9.060E−02, A₆ = 3.803E−02, A₈ =1.636E−04, A₁₀ = 2.065E−03, A₁₂ = −6.810E−03, A₁₄ = 6.321E−03, A₁₆ =−2.083E−03 Seventh Surface k = 0.000, A₄ = 1.571E−01, A₆ = −1.266E−01,A₈ = 8.500E−02, A₁₀ = −5.769E−02, A₁₂ = 1.034E−02, A₁₄ = 6.048E−03, A₁₆= −1.843E−03 Eighth Surface k = 0.000, A₄ = 2.079E−02, A₆ = 1.705E−02,A₈ = −1.366E−02, A₁₀ = 1.449E−03, A₁₂ = −1.039E−03, A₁₄ = 2.668E−04, A₁₆= 2.576E−04 Ninth Surface k = 0.000, A₄ = −1.522E−01, A₆ = 5.417E−02, A₈= −2.064E−02, A₁₀ = 1.903E−03, A₁₂ = −3.774E−04, A₁₄ = 1.755E−04, A₁₆ =−5.240E−07 Tenth Surface k = 0.000, A₄ = −1.234E−01, A₆ = 2.856E−02, A₈= −8.516E−03, A₁₀ = 8.785E−04, A₁₂ = 2.087E−04, A₁₄ = −5.150E−05, A₁₆ =2.699E−06 Eleventh Surface k = 0.000, A₄ = −1.011E−01, A₆ = 1.532E−02,A₈ = 2.440E−04, A₁₀ = 2.708E−05, A₁₂ = −5.790E−05, A₁₄ = 6.509E−06, A₁₆= −1.589E−07 Twelfth Surface k = 0.000, A₄ = −8.566E−02, A₆ = 2.416E−02,A₈ = −3.640E−03, A₁₀ = 2.008E−04, A₁₂ = 9.192E−06, A₁₄ = −1.708E−06, A₁₆= 6.523E−08 f1 = 3.76 mm f2 = 5.64 mm f3 = −4.07 mm f4 = 20.49 mm f5 =−103.21 mm f6 = −12.44 mm f56 = −12.38 mm F1 = 4.75 mm F2 = −37.47 mmThe values of the respective conditional expressions are as follows:f1/f2 = 0.67 F1/f = 1.09 f2/f = 1.29 f1/f3 = −0.92 F2/F1 = −7.90 D34/f =0.12 f5/f4 = −5.04 f56/f = −2.83 f6/f56 = 1.01 f6/f = −2.85

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.63 mm, and downsizing ofthe imaging lens is attained.

FIG. 14 shows a lateral aberration that corresponds to the image heightratio H, and FIG. 15 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively, of the imaging lens ofNumerical Data Example 5. As shown in FIGS. 14 and 15, according to theimaging lens of Numerical Data Example 5, the aberrations are alsosatisfactorily corrected.

Numerical Data Example 6

Basic data are shown below.

f = 4.42 mm, Fno = 2.2, ω = 37.7° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* 3.438 0.525 1.5346 56.1 (=νd1) 2* (Stop)−5.000 0.105  3* −6.491 0.716 1.5346 56.1 (=νd2)  4* −2.180 0.071  5*−10.432 0.279 1.6355 24.0 (=νd3)  6* 3.495 0.518 (=D34)  7* −2.260 0.6001.6142 26.0 (=νd4)  8* −2.082 0.091  9* 3.027 1.182 1.5346 56.1 (=νd5)10* 2.475 0.323 11* 9.088 0.358 1.5346 56.1 (=νd6) 12* 3.835 0.100 13 ∞0.300 1.5168 64.2 14 ∞ 0.731 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = −5.124E−02, A₆ = −6.397E−03, A₈ = −4.038E−02,A₁₀ = 2.150E−02, A₁₂ = 8.504E−03, A₁₄ = −1.982E−02, A₁₆ = 8.800E−03Second Surface k = 0.000, A₄ = −2.862E−02, A₆ = 1.387E−03, A₈ =−2.868E−02, A₁₀ = 8.664E−03, A₁₂ = 4.090E−02, A₁₄ = −5.307E−02, A₁₆ =2.202E−02 Third Surface k = 0.000, A₄ = 3.252E−02, A₆ = −1.802E−02, A₈ =2.463E−02, A₁₀ = −5.006E−02, A₁₂ = 3.297E−02, A₁₄ = −1.385E−02, A₁₆ =4.260E−03 Fourth Surface k = 0.000, A₄ = −1.717E−02, A₆ = −1.864E−02, A₈= 3.377E−04, A₁₀ = 4.360E−04, A₁₂ = −4.128E−04, A₁₄ = 1.749E−03, A₁₆ =−9.217E−04 Fifth Surface k = 0.000, A₄ = −1.669E−01, A₆ = 3.449E−02, A₈= 1.711E−02, A₁₀ = 1.272E−03, A₁₂ = 1.558E−02, A₁₄ = −1.450E−02, A₁₆ =3.060E−03 Sixth Surface k = 0.000, A₄ = −1.013E−01, A₆ = 3.107E−02, A₈ =−2.091E−03, A₁₀ = 2.399E−03, A₁₂ = −7.179E−03, A₁₄ = 6.162E−03, A₁₆ =−1.932E−03 Seventh Surface k = 0.000, A₄ = 1.764E−01, A₆ = −1.312E−01,A₈ = 9.052E−02, A₁₀ = −5.515E−02, A₁₂ = 6.803E−03, A₁₄ = 6.051E−03, A₁₆= −1.996E−03 Eighth Surface k = 0.000, A₄ = 1.544E−02, A₆ = 2.808E−02,A₈ = −1.245E−02, A₁₀ = 5.630E−04, A₁₂ = −1.402E−03, A₁₄ = 3.276E−04, A₁₆= 2.191E−04 Ninth Surface k = 0.000, A₄ = −1.589E−01, A₆ = 6.782E−02, A₈= −2.826E−02, A₁₀ = 5.799E−03, A₁₂ = −1.550E−03, A₁₄ = 3.395E−04, A₁₆ =2.769E−06 Tenth Surface k = 0.000, A₄ = −1.297E−01, A₆ = 2.914E−02, A₈ =−8.582E−03, A₁₀ = 9.292E−04, A₁₂ = 1.999E−04, A₁₄ = −5.198E−05, A₁₆ =2.927E−06 Eleventh Surface k = 0.000, A₄ = −1.069E−01, A₆ = 1.578E−02,A₈ = 4.037E−04, A₁₀ = 1.271E−05, A₁₂ = −5.888E−05, A₁₄ = 6.394E−06, A₁₆= −1.280E−07 Twelfth Surface k = 0.000, A₄ = −8.190E−02, A₆ = 2.405E−02,A₈ = −3.662E−03, A₁₀ = 2.028E−04, A₁₂ = 9.910E−06, A₁₄ = −1.747E−06, A₁₆= 6.099E−08 f1 = 3.90 mm f2 = 5.80 mm f3 = −4.09 mm f4 = 18.87 mm f5 =−100.43 mm f6 = −12.71 mm f56 = −12.74 mm F1 = 5.00 mm F2 = −57.38 mmThe values of the respective conditional expressions are as follows:f1/f2 = 0.67 F1/f = 1.13 f2/f = 1.31 f1/f3 = −0.95 F2/F1 = −11.48 D34/f= 0.12 f5/f4 = −5.32 f56/f = −2.89 f6/f56 = 1.00 f6/f = −2.88

Accordingly, the imaging lens of Numerical Data Example 6 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.80 mm, and downsizing ofthe imaging lens is attained.

FIG. 17 shows a lateral aberration that corresponds to the image heightratio H, and FIG. 18 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively, of the imaging lens ofNumerical Data Example 6. As shown in FIGS. 17 and 18, according to theimaging lens of Numerical Data Example 6, the aberrations are alsosatisfactorily corrected.

Numerical Data Example 7

Basic data are shown below.

f = 4.30 mm, Fno = 2.2, ω = 40.2° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* 3.481 0.517 1.5346 56.1 (=νd1) 2* (Stop)−5.004 0.131  3* −9.022 0.717 1.5346 56.1 (=νd2)  4* −1.978 0.039  5*−5.567 0.281 1.6355 24.0 (=νd3)  6* 4.042 0.577 (=D34)  7* −2.352 0.6001.6142 26.0 (=νd4)  8* −1.897 0.092  9* 2.617 0.705 1.5346 56.1 (=νd5)10* 2.620 0.458 11* −12.894 0.381 1.5346 56.1 (=νd6) 12* 3.288 0.100 13∞ 0.300 1.5168 64.2 14 ∞ 0.649 (Image ∞ plane) Aspheric Surface DataFirst Surface k = 0.000, A₄ = −5.529E−02, A₆ = −1.943E−03, A₈ =−4.003E−02, A₁₀ = 2.322E−02, A₁₂ = 1.073E−02, A₁₄ = −2.193E−02, A₁₆ =9.475E−03 Second Surface k = 0.000, A₄ = −2.363E−02, A₆ = 2.586E−03, A₈= −2.887E−02, A₁₀ = 1.906E−02, A₁₂ = 2.767E−02, A₁₄ = −5.102E−02, A₁₆ =2.631E−02 Third Surface k = 0.000, A₄ = 3.794E−02, A₆ = −2.509E−02, A₈ =2.200E−02, A₁₀ = −5.348E−02, A₁₂ = 2.091E−02, A₁₄ = −5.594E−03, A₁₆ =3.944E−03 Fourth Surface k = 0.000, A₄ = −1.782E−02, A₆ = −2.103E−02, A₈= 2.501E−03, A₁₀ = 2.279E−04, A₁₂ = −1.102E−03, A₁₄ = 8.723E−04, A₁₆ =−7.985E−04 Fifth Surface k = 0.000, A₄ = −1.498E−01, A₆ = 4.669E−02, A₈= 2.390E−02, A₁₀ = −7.375E−04, A₁₂ = 1.289E−02, A₁₄ = −1.463E−02, A₁₆ =3.616E−03 Sixth Surface k = 0.000, A₄ = −9.326E−02, A₆ = 3.885E−02, A₈ =−1.429E−04, A₁₀ = 1.505E−03, A₁₂ = −7.099E−03, A₁₄ = 6.277E−03, A₁₆ =−2.019E−03 Seventh Surface k = 0.000, A₄ = 1.374E−01, A₆ = −1.221E−01,A₈ = 8.381E−02, A₁₀ = −5.938E−02, A₁₂ = 9.481E−03, A₁₄ = 5.872E−03, A₁₆= −1.715E−03 Eighth Surface k = 0.000, A₄ = 3.155E−02, A₆ = 1.808E−02,A₈ = −1.385E−02, A₁₀ = 1.194E−03, A₁₂ = −1.112E−03, A₁₄ = 2.761E−04, A₁₆= 2.848E−04 Ninth Surface k = 0.000, A₄ = −1.570E−01, A₆ = 5.478E−02, A₈= −2.065E−02, A₁₀ = 2.264E−03, A₁₂ = −2.503E−04, A₁₄ = 1.755E−04, A₁₆ =−2.848E−05 Tenth Surface k = 0.000, A₄ = −1.209E−01, A₆ = 2.828E−02, A₈= −8.362E−03, A₁₀ = 9.025E−04, A₁₂ = 2.104E−04, A₁₄ = −5.160E−05, A₁₆ =2.613E−06 Eleventh Surface k = 0.000, A₄ = −8.595E−02, A₆ = 1.544E−02,A₈ = 2.164E−04, A₁₀ = 2.321E−05, A₁₂ = −5.827E−05, A₁₄ = 6.477E−06, A₁₆= −1.623E−07 Twelfth Surface k = 0.000, A₄ = −9.589E−02, A₆ = 2.497E−02,A₈ = −3.648E−03, A₁₀ = 1.962E−04, A₁₂ = 8.815E−06, A₁₄ = −1.715E−06, A₁₆= 6.812E−08 f1 = 3.92 mm f2 = 4.58 mm f3 = −3.64 mm f4 = 10.62 mm f5 =51.52 mm f6 = −4.86 mm f56 = −6.08 mm F1 = 4.69 mm F2 = −18.81 mm Thevalues of the respective conditional expressions are as follows: f1/f2 =0.86 F1/f = 1.09 f2/f = 1.07 f1/f3 = −1.08 F2/F1 = −4.01 D34/f = 0.13f5/f4 = 4.85 f56/f = −1.42 f6/f56 = 0.80 f6/f = −1.13

Accordingly, the imaging lens of Numerical Data Example 7 satisfies theabove-described conditional expressions except the conditionalexpression (13). The distance on the optical axis X from the object-sidesurface of the first lens L1 to the image plane IM (length without thefilter 10) is 5.44 mm, and downsizing of the imaging lens is attained.

FIG. 20 shows a lateral aberration that corresponds to the image heightratio H, and FIG. 21 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively, of the imaging lens ofNumerical Data Example 7. As shown in FIGS. 20 and 21, according to theimaging lens of Numerical Data Example 7, the aberrations are alsosatisfactorily corrected.

According to the imaging lens of the embodiment described above, it isachievable to have a wide angle of view (2ω) of 80° or greater.According to Numerical Data Examples 1 to 7, the imaging lenses havewide angles of view of 70.8° to 80.4°. According to the imaging lens ofthe embodiment, it is possible to take an image over a wider range thanthat taken by a conventional imaging lens.

Moreover, in these years, with advancement in digital zoom technology,which enables to enlarge any area of an image obtained through animaging lens by image processing, an imaging element having a high pixelcount is often used in combination with a high-resolution imaging lens.In case of such an imaging element with a high pixel count, alight-receiving area of each pixel decreases, so that an image tends tobe dark. As a method for correcting this problem, there is a method ofenhancing light-receiving sensitivity of the imaging element using anelectrical circuit. However, when the light-receiving sensitivityincreases, a noise component that does not directly contribute to imageformation is also amplified, so that it is necessary to use anothercircuit for reducing the noise. According to the imaging lenses ofNumerical Data Examples 1 to 7, the Fnos are as small as 2.2 to 2.4.According to the imaging lens of the embodiment, it is possible toobtain a sufficiently bright image without the above-describedelectrical circuit.

Accordingly, when the imaging lens of the embodiment is mounted in animaging optical system, such as cameras built in portable devicesincluding cellular phones, portable information terminals, andsmartphones, digital still cameras, security cameras, onboard cameras,and network cameras, it is possible to attain both high performance anddownsizing of the cameras.

The present invention is applicable to an imaging lens to be mounted inrelatively small cameras, such as cameras to be built in portabledevices including cellular phones, smartphones, and portable informationterminals, digital still cameras, security cameras, onboard cameras, andnetwork cameras.

The disclosure of Japanese Patent Application No. 2014-136552, filed onJul. 2, 2014, is incorporated in the application by reference.

While the present invention has been explained with reference to thespecific embodiment of the present invention, the explanation isillustrative and the present invention is limited only by the appendedclaims.

What is claimed is:
 1. An imaging lens comprising: a first lens grouphaving positive refractive power; and a second lens group havingnegative refractive power, arranged in this order from an object side toan image plane side, wherein said first lens group includes a firstlens, a second lens, and a third lens, said second lens group includes afourth lens, a fifth lens having negative refractive power, and a sixthlens, said third lens has a concave surface facing the image plane sidenear an optical axis thereof, said sixth lens has a concave surfacefacing the object side near an optical axis thereof, and said fifth lenshas an Abbe's number νd5 so that the following conditional expression issatisfied:40<νd5<75.
 2. The imaging lens according to claim 1, wherein said firstlens has a focal length f1 and said second lens has a focal length f2 sothat the following conditional expression is satisfied:0.3<f1/f2<0.9.
 3. The imaging lens according to claim 1, wherein saidfirst lens group has a focal length F1 so that the following conditionalexpression is satisfied:0.8<F1/f<1.2, where f is a focal length of a whole lens system.
 4. Theimaging lens according to claim 1, wherein said second lens has a focallength f2 so that the following conditional expression is satisfied:1.0<f2/f<2.0, where f is a focal length of a whole lens system.
 5. Theimaging lens according to claim 1, wherein said first lens has a focallength f1 and said third lens has a focal length f3 so that thefollowing conditional expression is satisfied:−1.2<f1/f3<−0.4.
 6. The imaging lens according to claim 1, wherein saidfirst lens group has a focal length F1 and said second lens group has afocal length F2 so that the following conditional expression issatisfied:−12<F2/F1<−1.5.
 7. The imaging lens according to claim 1, wherein saidthird lens is arranged away from the fourth lens by a distance D34 on anoptical axis thereof so that the following conditional expression issatisfied:0.05<D34/f<0.35, where f is a focal length of a whole lens system. 8.The imaging lens according to claim 1, wherein said fourth lens has afocal length f4 and said fifth lens has a focal length f5 so that thefollowing conditional expression is satisfied:−15<f5/f4<−5.
 9. The imaging lens according to claim 1, wherein saidfifth lens and said sixth lens have a composite focal length f56 so thatthe following conditional expression is satisfied:−3<f56/f<−0.8, where f is a focal length of a whole lens system.
 10. Theimaging lens according to claim 1, wherein said sixth lens has a focallength f6 so that the following conditional expression is satisfied:−3.5<f6/f<−0.5, where f is a focal length of a whole lens system.
 11. Animaging lens comprising: a first lens group having positive refractivepower; and a second lens group having negative refractive power,arranged in this order from an object side to an image plane side,wherein said first lens group includes a first lens, a second lens, anda third lens, said second lens group includes a fourth lens, a fifthlens having negative refractive power, and a sixth lens, said third lenshas a concave surface facing the image plane side near an optical axisthereof, said sixth lens has a concave surface facing the object sidenear an optical axis thereof, and said fourth lens has an Abbe's numberνd4 so that the following conditional expression is satisfied:15<νd4<35.
 12. The imaging lens according to claim 11, wherein saidfirst lens has a focal length f1 and said second lens has a focal lengthf2 so that the following conditional expression is satisfied:0.3<f1/f2<0.9.
 13. The imaging lens according to claim 11, wherein saidfirst lens group has a focal length F1 so that the following conditionalexpression is satisfied:0.8<F1/f<1.2, where f is a focal length of a whole lens system.
 14. Theimaging lens according to claim 11, wherein said second lens has a focallength f2 so that the following conditional expression is satisfied:1.0<f2/f<2.0, where f is a focal length of a whole lens system.
 15. Theimaging lens according to claim 11, wherein said first lens has a focallength f1 and said third lens has a focal length f3 so that thefollowing conditional expression is satisfied:−1.2<f1/f3<−0.4.
 16. The imaging lens according to claim 11, whereinsaid first lens group has a focal length F1 and said second lens grouphas a focal length F2 so that the following conditional expression issatisfied:−12<F2/F1<−1.5.
 17. The imaging lens according to claim 11, wherein saidthird lens is arranged away from the fourth lens by a distance D34 on anoptical axis thereof so that the following conditional expression issatisfied:0.05<D34/f<0.35, where f is a focal length of a whole lens system. 18.The imaging lens according to claim 11, wherein said fourth lens has afocal length f4 and said fifth lens has a focal length f5 so that thefollowing conditional expression is satisfied:−15<f5/f4<−5.
 19. The imaging lens according to claim 11, wherein saidfifth lens and said sixth lens have a composite focal length f56 so thatthe following conditional expression is satisfied:−3<f56/f<−0.8, where f is a focal length of a whole lens system.
 20. Theimaging lens according to claim 11, wherein said sixth lens has a focallength f6 so that the following conditional expression is satisfied:−3.5<f6/f<−0.5, where f is a focal length of a whole lens system.